The invention relates to a method and a device for the calibration of an image pick-up device which is sensitive to magnetic fields, and also to a method and a device for imaging by means of such an image pick-up device, notably an image intensifier in X-ray systems, for example, X-ray systems provided with a C-arm. The invention also relates to an X-ray system provided with devices of this kind.
The images acquired by image intensifiers in X-ray systems are liable to be distorted under the influence of external magnetic fields surrounding the image intensifiers. This problem is encountered notably when the image intensifier is attached to an arm (C-arm) which is rotatable about a patient. This is because it has been found that surprisingly the magnetic field strengths may deviate strongly in different locations within an examination room. This is due to the fact that the numerous electrical and metallic components of devices customarily present in such a room and equipment as well as shields which are made partly of a ferromagnetic material have a significant effect on the magnetic field in their vicinity. In this case the external magnetic fields have a different effect on the image distortions in every position of the image intensifier. Because of the magnetic fields which strongly vary within a room, the foregoing applies even more to mobile X-ray systems which can be displaced within the room.
EP 0 479 618 discloses a method and a device enabling the correction of magnetic and geometrical distortions in an X-ray image. To this end, a calibration is performed by means of a test object provided with a grid and a calibration table is created in which the calibration data determined is associated with each pixel position in the image. The images subsequently acquired are then corrected on the basis of this table.
However, this approach has a significant drawback in that, if the external magnetic field changes after the calibration, a new calibration must be carried out before each imaging operation. In the case of a mobile X-ray system which is used in various locations it is also necessary to perform a new calibration before each imaging operation.
Therefore, it is an object of the invention to provide a method and a device which enable comparatively simple and reliable calibration of an image pick-up device which is sensitive to magnetic fields.
It is also an object of the invention to provide a method and a device which enable such a calibration to be carried out in such a manner that it need not be repeated in the case of a varying magnetic field.
It is also an object of the invention to provide a method and a device which are suitable for the compensation of the effects of magnetic fields on the imaging in particular in mobile X-ray systems.
It is a further object of the invention to provide a method and a device for imaging which are conceived notably for use in conjunction with an image pick-up device calibrated in accordance with the invention and is optionally conceived also for surgical navigation.
Finally, it is an object of the invention to provide an X-ray system which is suitable in particular for mobile applications and in which the described effects of magnetic fields can be compensated for without requiring a significant amount of additional hardware.